She said they try to keep kids in class for every school day unless they're sick - and they don't intend to change that.

Prime Minister Jacinda Ardern, who fronted a live chat with students in Wellington this week, sympathised with the urgency behind the worldwide School Strike 4 Climate.

"Ultimately it's between students, parents and schools . . . but I think young people are genuinely worried about climate change and are looking to today's politicians to make the right decisions to combat it," she told theHerald.

The event is being backed by major education unions and dozens of leading academics, and the Government has left schools to decide what stance to take.

Secondary Principals' Association president Michael Williams said his own school, Pakuranga College, would mark any absences for the strike as "unjustified".

They would be subject to the standard process for truancy, he said, yet still assessed on a "case-by-case basis".

"For a child who has never taken a day off, and their parents have approved it, there is going to be a different level of scrutiny or consequence."

The country's second largest secondary school, Mt Albert Grammar, was also treating today as "business as usual", principal Patrick Drumm said.

"It's not a school-sanctioned event by any means and we can't guarantee [the students'] safety in terms of travel to and from school," he said.

"I'm not dismissing the critical nature of the discussion around this, and young people are very galvanised about it, which is fantastic, but we just aren't able to endorse students heading off on a school day."

Auckland Girls' Grammar School, meanwhile, has told parents that it acknowledged climate change was an important issue and asked them to advise if their children would be absent for the strike.

One of New Zealand's leading climate change experts, Victoria University glaciologist Professor Tim Naish, will be marching in Wellington to support his 15-year-old son, Henry.

"He likes this planet, but is pissed off with my generation for not taking action to look after it when the scientific evidence is so compelling."

Another student marching in Wellington, 17-year-old Watene Campbell of Te Kura Kaupapa Māori o Ngā Mokopuna, said his generation would "breathe, see and feel" the effects of climate change.

"I believe every person, be that student, teacher, or elder, should have the right to fight for what they believe in - especially on such an international scale."

"Disallowing students from attending the strike is not only a restriction of freedom of choice and speech, but also shows their inflexibility towards changing educational values accordingly to our changing world."

The Ministry for Education's deputy secretary for sector enablement and support, Katrina Casey, said schools might not consider students to be truants is they were "clearly engaged" in the strike.

"If on the other hand, a student simply uses the fact the action is happening to have a day off school they may be considered truant, as would be appropriate."

Both the New Zealand Post Primary Teachers' Association and New Zealand Educational Institute (NZEI) support the strike.

"Today's children will be the ones who have to live with the consequences of adults' apathy and inaction over the past few decades," NZEI president Lynda Stuart said.

"They're rightly angry, and it's about time politicians around the globe started thinking beyond the next electoral cycle."

MPs have also been divided over the strike.

Climate Change Minister James Shaw last week told theHeraldthat students would have to "weigh up the risks of being declared a truant, and any consequences that comes with that, against fighting for the future world that they will have to inhabit".

But Shaw said that he understood why they would want to strike, given previous generations had "failed comprehensively and let them down".

The protests are scheduled to be held in Aotea Square in Auckland's CBD from midday until 3pm; at Civic Square in Wellington at 10am, followed by a walk down Lambton Quay to Parliament; at Cathedral Square in Christchurch from 1pm and at George St in Dunedin, followed by a rally in the Octagon.

Events are also set for other centres including Hamilton, Palmerston North, Whanganui, Tauranga, New Plymouth, Lower Hutt and Nelson.

Student: Five Reasons Why I'm Striking

"I'm striking because I'm concerned about the future of myself, my children and the younger generations to come," 18-year-old Aaron Choi says. Photo / Supplied

Auckland International College student Aaron Choi gives five reasons why he's taking part in today's strike.

1.It is so easy for the world to turn a blind eye towards global issues such as climate change because we are all caught up and busy in our own lives, especially when we enter the adult world. I am striking as a high school student to remind my school peers, our generation and especially myself to never forget about the work that needs to be done when we enter the adult life after high school.

2.Although I am currently a student, I am also striking as a future worker of this world. Educating and developing myself is futile in a dying world where humanity will perish unless action is taken now.

3.I'm striking to feel more connected with the conservation community here in New Zealand, hoping to meet like-minded people and collectively show the rest of the world that a community like us exists.

4.I'm striking to start an open conversation about climate change not just with our government, but the rest of the world, and others around us who do not believe in our cause.

5.I'm striking because I'm concerned about the future of myself, my children and the younger generations to come.